Fuel cells are electrochemical devices in which electricity is produced by a direct reaction of a fuel and an oxidizer. Fuel cells are highly efficient, nonpolluting and silent in operation. Hence, fuel cells are coming into greater and greater use.
In the operation of a fuel cell, electrochemical oxidation and/or reduction reactions take place at the surface of the electrodes of the cell. Rates of the reactions increase with temperature, and as such there is a motivation to increase the operating temperature of fuel cells in order to enhance the reaction kinetics as well as electrode tolerance to species such as carbon monoxide, which are byproducts of fuel processing and which can operate to corrode, poison or otherwise degrade the electrode surface.
In conventional fuel cells, the membranes separating the electrodes rely heavily on water as a proton charge carrier. This reliance limits the operating temperature range of such fuel cells to less than 100° C. at atmospheric pressure. If higher temperature operation of such cells is desired, they must be pressurized. Even at temperatures below the boiling point of water it is necessary to maintain an adequate water balance in the fuel cell. Therefore, an additional humidification system and hydrating water are often required, and this increases the complexity of the fuel cell system and reduces the energy density.
In order to overcome these limitations associated with conventional fuel cell membranes, two classes of intermediate temperature (150-200° C.) electrolytic membranes have been proposed. The first class uses high thermostability polymers doped with an acid, and the second class is based on the use of solid acids such as cesium hydrogen sulfate. However, it has been found that problems are associated with both of these types of membranes. In particular, the acid-doped polymeric membranes contain high concentrations of unbound acid molecules that limit the fuel cell stability due to leaching of acid from the membrane. This leaching problem is extremely detrimental, particularly in small, portable fuel cell units where water may condense on the surface of the polymer at low temperatures during the time the fuel cell is out of service. The solid acid membranes have similar hydrolysis and stability limitations in the presence of liquid water. Furthermore, solid acid electrolytes suffer from processing difficulties that have limited membrane thickness to the millimeter range. This increases the parasitic resistance of the fuel cell and reduces its efficiency.
In view of the problems with prior art membranes, there is a need for fuel cell membranes that can sustain high and stable conductivity at temperatures above 150° C. without requiring additional humidification systems or hydrating water. As will be detailed hereinbelow, the present invention provides for the manufacture of fuel cells which include polymer electrolyte membranes fabricated from a novel class of polyimidazole polymers. These membranes have good conductivity for protons and can operate at ambient pressure utilizing liquid fuel such as methanol. These and other details of the invention will be apparent from the discussion and description which follow.